1. Field of the Invention
The present invention relates generally a large circular microwave cavity device capable of supporting higher transmission modes with a resonance absorber for suppressing unwanted modes.
2. Description of the Related Art
Several types of microwave devices have large circular regions capable of supporting higher transmission modes, an example would be a circular combiner (or divider) that ties many inputs into a singular output. In the circular combiner, the coaxial inputs are spaced around the circumference of a flat circular cavity at an equal radius from the central coaxial output. The cavity consists of a top and bottom,ground plane spaced by air or other dielectric. Although the circular combiner is large enough to propagate many modes, it depends upon symmetry in the location of the inputs, voltage, and phase to avoid generating anything but the simplest uniform mode.
Another device capable of propagating many modes is the "pancake" rotary joint which requires a large diameter central hole and consequently a large diameter coaxial path. Moding, the presence of one or more undesired modes or resonances, is avoided in the "pancake" rotary joint by driving it with equal phase and amplitude at many points around the circumference. If there is an imperfect amplitude or phase, higher modes will be excited.
Devices, such as the "pancake" rotary joint, are several wavelengths in circumference and will propagate higher order modes that do not have circular symmetry. Unbalance in drive or lack of in symmetry in manufacture will inject energy into these modes, and couple energy out of them. The path lengths are very different in the various modes thereby causing their signals to add and subtract with each other thus causing variations in impedance and losses as a function of rotation. A resonance also occurs when a weakly coupled mode has a path length that is an integral number of wavelengths.
In the past, an attempt has been made to correct the problem of maintaining an equal drive by the use of terminated couplers in the circuit. However, this requires the adding of high power loads to the circuit and a means of transferring the load heat to the outer body which adds considerably to the size and weight of the device. An alternate solution is to slot the body of the device between the drive points. This interrupts the unbalanced modes and results in a current along the rim of the coupler that is intercepted by the absorber in the slots. However, the body structure of the device is weakened because the slot between the drive points must be a quarter wavelength deep.
For many applications, even folding the slot is inadequate because cutting into the circumferential wall weakens the structure at a location where the utmost of accuracy is required. However, in the design of a low frequency rotary coupler, none of the above approaches have been found to be practical because there is insufficient room for either isolation couplers or quarter wavelength slots in the body because at the operational frequency of the device the slots would be many inches in length. Also, using a folded slot unacceptably weakened the structure and interferes with the desired mode. At low frequencies, such as 1 GHz, such a slot would be three inches deep and therefore unacceptable.